1. Technical Field
The present invention relates to the purification of liquids, including water, milk and the like, utilizing an air floatation basin and a skimming device for removal of floating sludge. The skimming device reciprocally transports floating sludge contained in such floatation basins, often used, e.g., at water purification and waste water treatment plants.
The device of the invention has one or more skimming blades and one profile-following, preferably flexible-edged skimming blade which raises the sludge above the liquid level line thereby removing excess liquid just prior to depositing the sludge into a collecting hopper for removal and disposal. The apparatus is manufactured of corrosion resistant materials, has few moving parts, produces a relatively low percentage of liquid in the removed sludge and is easily operated and serviced.
2. Description of the Prior Art
Floatation methods have been applied in purification of liquids, especially water, and are mainly used when high purification of the impure water is required. The floatation is carried out in a floatation basin wherein the impurities floated out collect as a scum or sludge on the surface of the liquid, whereupon the scum is removed by scoops or with the aid of suction means. The floatation basin may be of various shapes, but those generally used are cylindrical or rectangular.
Dissolved air floatation basins produce large quantities of floating sludge which must be removed from the basins at set intervals. These basins are normally constructed to a smaller scale than conventional gravity settling basins due to their effectiveness in removing large quantities of chemically flocculated particulates from the water purification plant influent. Floated dissolved air floatation sludge is removed from the basin by utilizing a skimming mechanism to shift the sludge and deposit it into a sludge hopper located at the effluent end of the basin. An objective of such devices is to remove the floated dissolved air floatation sludge at the highest possible dry solids content in order to decrease the volume of sludge for direct disposal or further dewatering processes, thus decreasing disposal and further dewatering process expenses. These basins may utilize an inclined or upwardly curved ramp at the sludge hopper end of the basin. This ramp is used to separate the water elevation and the hopper to prevent an excess of purified water from escaping into the sludge hopper. The skimming mechanism must traverse this ramp in order to deposit the sludge into the hopper, thereby necessitating the need for the skimming blade profile' s line of action to follow the contour of the ramp. In order for the ramp to provide the maximum dewatering of the sludge, the ramp must be built of a specific radius or slope. The longer the sludge remains on the ramp, the more effective is the dewatering of the sludge. Thus, by building a ramp with a large diameter or a gradual slope, the skimming mechanism can retain the sludge on the ramp above the purified water level for a set period of time to provide thorough sludge dewatering before complete removal of the sludge into the sludge hopper.
Chain driven skimmers have been used previously in many dissolved air floatation basins to remove the sludge which, little by little, floats to the liquid surface. These chain driven skimmers consist of one or more skimming blades which extend across the basin and which are towed forwardly by means of continuously working chains and sprockets. These constructions have certain advantages by providing a good cleansing result and have in other respects a good functionality security as long as the construction is intact. In spite of its solid basin construction, however, serious breakdowns often occur due to the tremendous corrosion which all components, including the chains, are subjected to because the environment in the settling basins of the purification plants is very corrosive. Chain driven skimmers, moreover, require a large distance between water surface and walking bridges which leads to high building costs and clearance problems in existing facilities. The use of chain type skimmers dictates the curvature or slope of the ramp located at the sludge hopper end of the basin. This curvature or slope is not always adequate for thorough dewatering of the sludge before it is completely removed. In order to increase the curvature or slope of the ramp, taller sprockets must be employed which increases the building height even further, adding significantly to the expense of the installation.
Thus there exists a need for a type of skimmer device to thoroughly dewater and remove dissolved air floatation sludge by means of a skimming blade capable of duplicating its line of advance along a radius or mildly sloped, upwardly extending ramp.